The present invention relates to an optical range finder or distance measuring device used for a camera.
An optical device provided in a camera uses a method such as an active method or a passive method for measuring a distance. In the active method, light is emitted toward an object, and a distance to the object is determined with triangulation. In the passive method, light irradiated from an object is detected, and a distance to the object is determined through a difference in phases of the light.
The active method will be described below in detail. An active range finder includes a light emitting unit having a light emitting element such as a light emitting diode (LED), a light emitting lens, a light receiving lens, and a light receiving unit having a light detecting element such as a position sensitive detector (PSD). The light emitting unit emits light toward an object through the light emitting lens, and the light receiving unit receives the light reflected from the object through the light receiving lens.
The light detecting element outputs an electrical signal according to a position of a light detecting plane where the light reflected from the object irradiates. Accordingly, it is possible to determine an angle of the incident light emitted through the light emitting lens based on the electrical signal according to the position of the light detecting plane. Therefore, it is possible to determine a distance to the object through the principle of triangulation using the incident angle and a distance between the light emitting lens and the light receiving lens.
An example of the passive method will be explained next in detail with reference to FIGS. 11(a)–11(c) and 12. FIG. 11(a) is a top plan view of a passive range finder. FIG. 11(b) is a cross sectional view taken along line 11(b)—11(b) in FIG. 11(a). FIG. 11(c) is a side view of the passive range finder shown in FIG. 11(a). FIG. 12 is a diagram for explaining the principle of the passive method. As shown in FIGS. 11(a) through 11(c), a passive range finder 500 includes a package 501, leads 502, a semiconductor chip 503, bonding wires 504, an optical casing 505, and a lens unit 506.
As shown in FIGS. 11(a) and 12, the lens unit 506 includes a first lens 506L and a second lens 506R arranged side by side with a certain distance b therebetween. As shown in FIG. 11(a), the first lens 506L and second lens 506R are integrated into the lens unit 506, for example, by resin molding.
As shown in FIG. 12, a semiconductor chip 503 includes a first light detecting section 503L and a second light detecting section 503R. The first and second light detecting sections 503L and 503R include many light detectors such as complementary metal oxide semiconductors (CMOS) and charge coupled devices (CCDs) arranged side by side. The passive range finder 500 also includes an A/D converting unit 507, an operating unit 508, and a CPU 509 for processing a signal. The A/D converting unit 507, operating means 508, and CPU 509 connected to each other via leads 502 process the signal outputted from the semiconductor chip 503 for determining a distance to an object.
The passive method for determining a distance will be explained next. As shown in FIG. 12, the first and second lenses 506L and 506R are spaced apart by a reference distance b. The first and second light detecting sections 503R and 503L are mounted on a surface of the semiconductor chip 503, and the semiconductor chip 503 is disposed at a location away from the first and second lenses 506L and 506R by a focal length f of the first and second lenses 506L and 506R. The light from the object is focused on the first light detecting section 503L through the first lens 506L and on the second light detecting section 503R through the second lens 506R, so that an image of the object away from the range finder 500 by a distance d is obtained.
When the object is located at an infinite distance from the range finder 500, two parallel rays io passing through the first and second lenses 506L and 506R are incident on reference points PO on the first and second light detecting sections 503L and 503R. When the object is located at a finite distance d from the range finder 500, an image of the object is formed at a point PL on the first light detecting section 503L shifted from the reference point PO by a displacement XL and at a point PR on the second light detecting section 503R shifted from the reference point PO by a displacement XR. In this case, the distance d is expressed by equation (1) using the displacements XL and XR.d=b×f/(XL+XR)  (1)
The reference distance b and the focal length f are known characteristics of the range finder 500. Accordingly, the distance d is obtained from the displacements XL and XR of the image of the object formed on the first and second light detecting sections 503L and 503R.
Based on the principle of the distance measurement described above, each light detecting device outputs a signal according to an intensity of the light, and the range finder 500 outputs an image signal of the object. The A/D converting unit 507 conducts A/D conversion of the image signal and outputs image data. The operating unit 508 processes the image data to obtain the distance d, and outputs distance data. The CPU 509 conducts control operations such as displaying the distance d on a display (not shown) using the distance data. Accordingly, the passive range finder determines a distance to an object based on the principle as described above.
In the range finder using the conventional technique, a measured distance tends to have an error due to thermal expansion and thermal contraction caused by a temperature change (hereinafter the thermal expansion and thermal contraction caused by a temperature change will be referred to collectively as the “thermal expansion and contraction”). An error of the measured distance will be referred to as “measured distance error.” The measured distance error will be described in detail below with reference to FIG. 13. FIG. 13 is a diagram for explaining a cause of the measured distance error.
In the conventional range finder, a lens distance b between optical axes of the first and second lenses 506L and 506R changes due to the thermal expansion and contraction. A distance between reference elements on the first and second light detecting sections 503L and 503R is also changed by the thermal expansion and contraction. Suppose that the lens distance is equal to bO at a temperature tO in an initial state, and a thermal expansion coefficient of a material of the lens is τL. When the temperature changes by Δt, the lens distance b between the first and second lenses 506L and 506R is described by equation (2).b=bO+bO×Δt×τL  (2)
The term bO×Δt×τL represents an increment or a decrement of the lens distance caused by the thermal expansion and contraction. When the semiconductor chip has a thermal expansion coefficient τC, the distance between the first and second light detecting sections 503L and 503R (distance between the reference points PO changes as described by equation (3).b=bO+bO×Δt×τC  (3)
As described above using equation (1), the distance d to the object is determined using the lens distance b between the first and second lenses 506L and 506R. Therefore, when the lens distance b and the detecting device distance b change relatively, the displacements XL and XR from the reference points PO change, thereby causing the measured distance error.
The measured distance error will be explained next. When the distance between the first and second lenses 506L and 506R is bO and the focal length of the lenses is fO at the temperature to in the initial state, the distance d to the object is expressed by equation (4) according to equation (1).d=b0×f0/(XL+XR)  (4)
The initial lens distance bO and the initial focal length fO of the lenses are known constants. When the temperature of the range finder 500 changes by Δt, the term b0×f0 in equation (4) is changed accordingly. The change of the term b0×f0 is small relative to a value of the term b0×f0 within a temperature change Δt of several tens of degrees. On the other hand, the lens distance b of the lenses and the detecting device distance b of the semiconductor chips are seriously affected by the thermal expansion and contraction.
When the temperature is changed by Δt, the distance d′ is approximately expressed by equation (5), where the thermal expansion coefficient of the lens material is τL and the thermal expansion coefficient of the semiconductor chip material is τC.d′=b0×f0/{XL+XR−b0×Δt(τL−τC)}  (5)
As can be seen in equation (5), the temperature change Δt has a significant effect on the term b0×Δt(τL−τC). As a result, the measured distance d′ is greatly deviated from the actual distance d. An example of the measured distance error will be described in detail below.
Suppose that the temperature in the initial state is 25° C., the temperature change Δt is +20° C., the lens distance bO is 5.5 mm, the focal length of the lenses fO is 6.0 mm, the thermal expansion coefficient τL of the lens made of polycarbonate is 7.0×10−5/° C., the thermal expansion coefficient τC of the semiconductor chip made of silicon is 2.5×10−6/° C., and the actual distance d to the object is 1000 mm. The total displacement (XL+XR) is given as follows using equation (1):(XL+XR)=0.033 mm  (6)
The temperature change Δt creates an influence expressed as follows:b0×Δt(τL−τC)≈0.074 mm  (7)
The distance d′ after the temperature change is calculated using these values to be about 1290 mm. The measured distance d′ is greatly deviated from the actual distance d due to the temperature change. To reduce the adverse influence of the temperature change in the range finder, various techniques have been considered.
Japanese Patent No. 3310079 has disclosed a technique in which range finders having two different lens distances, i.e. large and small, are combined and switched depending on a temperature to reduce the measured distance error.
Japanese Patent Publication No. 10-232128 has disclosed a range finder having a temperature sensor disposed in an instrument housing for measuring a temperature at every operation. In the range finder, a relationship between the measured distance and the temperature change is determined in advance, so that the measured distance error due to the temperature change is predicted.
Japanese Patent Publication No. 10-232128 has disclosed a technique in which a lens is positioned and fixed to a lens holder such that expansion or contraction of the lens is reduced when the lens holder is expanded or contracted due to an environmental change.
The conventional techniques described above have various problems. In the technique disclosed in Japanese Patent No. 3310079, the measured distance error is not essentially reduced. Further, it is necessary to provide two range finders and a space for the two range finders inside the instrument such as a camera, thereby making the range finder not practical.
The range finder disclosed in Japanese Patent Publication No. 10-232128 has the temperature sensor disposed in the vicinity of the range finder, and it is necessary to send the temperature data to a control system for correction in response to the temperature. Accordingly, it is necessary to provide the temperature sensor and an extra circuit for receiving the temperature data, and to conduct the complicated correction control, thereby increasing cost, the number of electronic circuits and control operations.
In the technique disclosed in Japanese Patent Publication No. 10-232128, it is possible to reduce the expansion or contraction of the lens. However, it is difficult to completely eliminate the expansion or contraction of the lens. Accordingly, it is necessary to provide a device for measuring a temperature as described above to reduce the expansion or contraction of the lens with high accuracy.
In view of the problems described above, the present invention has been made, and an object of the present invention is to provide a range finder with a simple structure capable of reducing the adverse influences of the thermal expansion and contraction.
Further objects and advantages of the invention will be apparent from the following description of the invention.